1. Field of the Invention
The invention relates to an annular combustion chamber of a turbomachine, of the type including an inner wall, an outer wall, a chamber bottom disposed between said walls in the upstream region of said chamber, and two attachment flanges disposed downstream of the chamber bottom and respectively enabling said walls to be attached to other parts of the turbomachine, generally inner and outer casings surrounding the combustion chamber.
2. Description of Related Art
Formerly, said inner and outer walls of the chamber were made of metal or metal alloy and it was necessary to cool these walls to enable them to withstand the temperatures reached during operation of the turbomachine.
Today, so as to reduce the air flow allocated to the cooling of these walls, the latter are made of ceramic material rather than metal. Ceramic materials are effectively better at withstanding high temperatures and have a lower bulk density than the metals customarily used. The gains made in terms of cooling air and weight result in improved efficiency of the turbomachine. It will be noted that the ceramic materials used are, preferably, ceramic matrix composites chosen for their good mechanical properties.
With regard to the chamber bottom and the attachment flanges, the state of the technology requires that these components be made of metal or metal alloy, rather than ceramic material, thereby facilitating the use of known and proven fixing methods making it possible to fix the attachment flanges to the metallic casings of the combustion chamber and the injection systems to the chamber bottom. These fixings can be made, for example, by welding or bolting.
The ceramics used to make the walls often have a coefficient of expansion around three times lower than that of the metallic materials used to make the chamber bottom and said flanges. A difference of this magnitude generates stresses in the assembled components during the assembly thereof, and also when their temperature rises in operation. These stresses can be the cause of cracking in the attachment flanges or in the walls, (if the flanges are not sufficiently flexible), the ceramic materials being rather brittle by nature.
To remedy this problem, a solution described in the document FR 2 855 249 consists in providing a plurality of flexible fixing lugs connecting the chamber bottom to said walls, these lugs being capable of deforming elastically in relation to the differential expansion between the components.
Other known solutions are described in patent applications FR 2 825 781 and FR 2 825 784, which consist in connecting the walls to the casings of the combustion chamber by means of several resiliently deformable flexible fixing lugs replacing the annular attachment flanges.
In all of these prior art documents, the inner and outer walls of the combustion chamber are made in one piece of generally conical shape.
The principal drawback of known structures with flexible fixing lugs lies in the poor dynamic behaviour of these fixing lugs, during operation of the turbomachine, and it is often necessary to provide damping systems to limit the deformation of these lugs and the vibration generated.
Moreover, in FR 2 855 249, there remains between the fixing lugs, at the level of the chamber bottom, spaces into which fresh air rushes, which can degrade the efficiency of the combustion chamber by promoting the formation of polluting emissions such as, for example, incomplete combustion products and/or carbon monoxide.